It is known that the amount of NOx produced from an internal combustion engine which combusts (burns) gasoline as a fuel depends on a combustion period of time during which the temperature in a combustion chamber is high temperature, and becomes smaller as the combustion period of time shortens. In premixed-charge compression auto-ignition (self-ignition) combustion in which an air/fuel mixture formed in the combustion chamber is compressed at high compression ratio to thereby be ignited, the compressed air/fuel mixture is ignited substantially simultaneously at a large number of scattered positions. Accordingly, auto-ignition combustion completes within a shorter period of time than spark ignition combustion in which fuel is combusted through propagation of flame. Thus, an internal combustion engine that carries out auto-ignition combustion can reduce NOx emissions as compared with an internal combustion engine that carries out spark ignition combustion. Furthermore, auto-ignition combustion enables combustion at high compression ratio and at very lean air/fuel ratio, so that the fuel efficiency is improved.
Since auto-ignition combustion completes within a very short period of time, auto-ignition combustion in a high-load region, which requires a large amount of fuel, causes an abrupt increase in pressure (cylinder pressure) within the combustion chamber, and therefore, very loud combustion noise may be caused. In order to cope with the noise problem, there has been developed a 4-cycle internal combustion engine which performs an auto-ignition combustion operation in a light-load region where the auto-ignition combustion causes low noise; and which performs a spark ignition combustion operation in which an air/fuel mixture is ignited by a spark generated by a spark plug, in a high-load region where loud combustion noise is generated by the auto-ignition combustion (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. 2000-64863, claim 1 and FIG. 4).
Meanwhile, a compression ratio of an internal combustion engine designed to carry out the auto-ignition combustion is considerably higher than a compression ratio of an internal combustion engine designed to carry out the spark ignition combustion. Accordingly, if the internal combustion engine which can perform auto-ignition combustion performs spark ignition combustion when the load is high, knocking arises. In this connection, the conventional internal combustion engine, in order to lower actual compression ratio, delays timing of closing of an intake valve until near the top dead center of a compression stroke in a spark ignition operation, so that knocking can be avoided.
However, since operation in a high-load region requires combustion of a large amount of fuel, there is a limit imposed on the extent of delaying timing of closing of an intake valve. This is because excessively delaying timing of closing of an intake valve causes a reduction in the amount of an air/fuel mixture (i.e., amount of fuel). Thus, the conventional internal combustion engine can not avoid knocking sufficiently and can not increase torque which the engine generates.